Luther and Lewis

Those who serve God out of a good, firm will are steadfast in His service, whether things go well or ill, whether they are sweet or bitter, for they have been made firm by God with a noble, princely, unconstrained free will; for the little word, 'free spirit' which stands here means in Hebrew a favourable, unconstrained spirit. All that is done by force, does not last; but what is freely willed remains firm.—Martin Luther, Seven Penitential Psalms

It has not pleased God that a distinction between a sin and a duty should turn on fine feelings. This act [of Eros], like any other, is justified (or not) by far more prosaic and definable criteria; by the keeping or breaking of promises, by justice or injustice, by charity or selfishness, by obedience or disobedience.—C. S. Lewis, Business of Heaven, p. 164